1. Field of the Invention
The present invention relates to a gas sensor including a cylindrical metal shell extending in an axial direction and a detection element main body extending in the axial direction and attached at the diametrical inner side of the metal shell, and in particular, to a gas sensor including a porous protection layer that covers a leading end side in the axial direction of the detection element main body.
2. Description of the Related Art
The use of a gas sensor attached to an exhaust pipe of an automobile and including a detection element configured to vary an electromotive force or a resistance value according to the concentration of a specified component (for example, nitrogen oxide, oxygen, etc.) of exhaust gas is known.
The leading end portion of the detection element main body is coated with a porous protection layer including porous ceramic so as to prevent cracking that can occur due to thermal shock generated when water drops in exhaust gas adhere to the detection element. The porous protection layer disperses and slowly absorbs the water drops adhering to the detection element. Therefore, a large thermal shock is hardly generated at the detection element main body, and it is thus possible to prevent cracking of the detection element main body. For example, JP-A-2007-33374 (FIG. 1 and related description of this reference) discloses a gas sensor having such a protection layer.
However, the base end of the porous protection layer has a sharp corner part which may chip when subjected to vibration and shock in use. When water drops in exhaust gas adhere to a thinned or exposed portion of the porous protection layer due to such a chip, a crack may occur at the thinned or exposed portion of the detection element main body.